A step-out detection device of a sensorless synchronous motor has been proposed in the related art (for example, Patent Document 1). A driving device of the sensorless synchronous motor applies a three-phase AC voltage to stator windings, generates torque by using the interaction between current flowing through the stator windings and magnetic fields caused by permanent magnets of a rotor, and then is rotated. The step-out detection device of the sensorless synchronous motor includes the structure that detects step-out when a detection level of an effective value of current of the stator windings is set, the effective value of the current of the stator windings exceeds the detection level, and a power factor angle between a voltage applied to the stator windings and the current flowing through the stator windings is close to 90°.
Patent Document 1: JP-A-9-294390 (see pages 2 to 4, FIGS. 1 and 2)